A radio frequency switch, short: RF switch, is a device to route an RF signal through a transmission path. An RF switch is used extensively in a microwave test system for an RF signal routing between an instrument, such as a signal generator or a measuring device, and a device under test, short: DUT. Incorporating an RF switch into a RF switch matrix system enables to route an RF signal from single or multiple instruments to single or multiple DUTs. This allows multiple tests to be performed with the same setup, eliminating the need for frequent connects and disconnects. Similar to an electrical switch, an RF switch comes in different configurations providing the flexibility to create complex matrices and automated test systems for many different applications.
There are single-pole-double-throw RF switches, short SPDT-switches that route an RF signal from one input terminal to two output terminals. There are single-pole-multiple-throw RF switches, short SPnT-switches that route an RF signal to multiple, at least three or more, output terminals.
Alternatively, an RF switch might be employed to selectively connect an antenna of a communication device with either transmit or receive circuitry of the device or with a signal path among a plurality of other components.
An RF switch should be designed to not influence the RF signal while routing the RF signal. Therefore, an RF switch is characterized by its frequency range, its insertion loss, its return loss, its repeatability, its isolation, its switching speed, its setting time, its power handling, its termination, its video leakage, its non-linearity and its operating life. Those parameters of an RF switch are used to classify the RF switch for a specific application.
Nowadays, an RF switch utilizes at least one field effect transistor, short FET, as a switching device. A FET comprises excellent parameter values to not influence the RF signal during routing. Thus, proper values for the above cited RF switch characteristics are obtained when using a FET. When operating a FET in an RF switch, the drain destination and the source destination of the FET are typically not meaningful as the FET is operated in a symmetric fashion.
A switching device according to the invention is preferably used as a shunt element in an RF switch. In electronics, a shunt element is a device which allows electric current to pass around another point in the circuit by creating a low resistance path.
In general, there are a number of parasitic capacitances associated with a FET used as a switching device in an RF switch arising from the FET itself. Additionally, various resistive and capacitive components employed in the RF switch or electrical traces connected to the RF switch might lead to the parasitic capacitances as well. These capacitances can degrade various aspects of the FET switching performance. In particular, a parasitic gate capacitance to ground degrades the linearity of the FET switch and results in degraded harmonic and intermodulation performances for the FET in the RF switch. Additionally, harmonic distortion occurs, which is a main effect on the transistors capacitances.
In US 2011/0233628 A1, a FET structure as a switching device for RF signals is described. To avoid a parasitic capacitance from the gate to the ground, it is proposed to connect a drain node to a gate node via a series connection of a resistor and a capacitor to obtain a high frequency conducting bridge. Thus, the gate node follows the RF signal applied to the drain node.
A drawback of this technique is the fact that the high frequency bridge is not working with low frequency signals. At low frequency signals, the series connection from the gate node to the drain node becomes highly resistive and blocks the RF signal. Additionally, harmonic distortions of the RF signals generated by the parasitic capacitances of the FET cannot be avoided with such a structure.
When using such a FET structure as a shunt element in an RF switch, the FET channel is asymmetrically connected to a specific voltage potential, preferably ground potential of a supply voltage. Such an asymmetric arrangement to ground increases the parasitic capacitance of the FET and leads to an unwanted non-linear distortions of an RF signal while routing.
Thus, there is a need to provide an RF switching device that is flexible in its use and can be operated in a great variety of applications. The RF switching device should at least operate in a frequency range from DC to microwave frequencies. The parasitic capacitance of the RF switching device and especially the harmonic distortion should be further reduced.